Digital communications equipment conventionally produces an analog sinusoid for transmission of information. This analog sinusoid is conventionally utilized in the up-conversion stage of a transmitter and the down-conversion stage of a receiver.
Others have constructed such an analog sinusoid waveform using digital techniques to generate a discrete version of the target signal and then provide the analog sinusoid waveform using a digital-to-analog converter (“DAC”) and, post-DAC, an anti-imaging filter. Known examples of anti-imaging filters include Bessel, Elliptic, Butterworth, and Chevychev, among others. Prior to the DAC, a digital sinusoid signal may be realized using a look-up table (“LUT”) memory. Accordingly, others have coupled an accumulator, quantizer, LUT memory, DAC, and anti-imaging filter in series as a frequency synthesizer architecture for Direct Digital Synthesis (“DDS”). For a bandwidth of interest, the spectral region over which such analog sinusoid is to be generated may be a fraction of the entire Nyquist (“sample frequency”) bandwidth.
The quality of an analog waveform of a DDS system may be defined at least in part by the spurious free dynamic range (“SFDR”) of a sinusoid signal produced. This metric defines the difference between amplitude of the highest spurious signal and a target sinusoid component or signal of the spectra produced. Accordingly, it would be desirable to suppress amplitude of spurious signals or tones of a spectra to enhance the SFDR. However, suppression of spurious tones conventionally involves increasing the address bus of the LUT memory, and this means that the amount of memory used is correspondingly increased. Additionally, to enhance quality of the digital-to-analog conversion, the DAC may likewise conventionally need to be able to handle larger sample sizes obtained from the LUT. Either or both increased memory size for the LUT or increased capability of the DAC leads to a significant increase in overhead.